This invention relates to static trip circuit interrupters in general, and in particular, to those type interrupters employing current transformers for both sensing current magnitudes through a protected circuit as well as for providing operating power to the static trip circuit interrupter components. One such circuit is described within U.S. Pat. No. 4,266,259 to E. K. Howell, which patent is incorporated herein for purposes of reference. During the manufacturing process of the Howell static trip unit, the current transformers are calibrated by providing known values of current to the transformers and the transformer circuit is adjusted accordingly. During long term operation of the static trip unit, the transformer impedance may vary depending upon use and climatic conditions, such that the transformers may provide output currents which differ slightly from the original values at calibration.
To provide for exact determinations of the long time and short time pickup values, the timing resistors within the long term trip mode network and the short time trip mode network must be accurately trimmed by a laser trimming process which involves sophisticated equipment and skilled equipment operators. The same is true for trimming the resistors that are used within the voltage regulator portion of the trip unit power supply.
As common with many static trip units, semiconductor elements such as diodes and transistors are used as discrete elements along with the integrated circuit components within the signal processor boards. These semiconductor elements can be temperature variant and some means should be provided to compensate for seasonal variations in temperature.
It would be desirable to provide a correction factor for variations in the current transformer output over a wide range of operating currents and to calibrate the precision resistors required within the long time and short time trip mode networks as well as to incorporate a correction factor for the semiconductor components, both discrete and integrated, with no loss in manufacturing time and without requiring expensive calibration and trimming equipment.
The purpose of this invention is to provide such a calibration and correction facility to static trip units which insure accurate and precise readings throughout wide variations in the operating currents and/or seasonal temperatures throughout the operational life of the trip unit.